Optical measurements are becoming increasingly important for analysis within the pharmaceutical industry. Spectroscopy offers the obvious advantages of fast, non-destructive, non-invasive and flexible methods well applicable for analysis near or in the production line. In this context, near-infrared (NIR) spectroscopy is a well-established technique for qualitative and quantitative analysis of the active component and excipients in many different products. In parallel, spectroscopic techniques for measuring structural parameters of pharmaceuticals have been developed; in particular light scattering methods are well known techniques for determination of particle size distribution of powders and solutions. However, determination of physico mechanical parameters of a solid or semi-solid sample is more complex than analysis of chemical content. In fact, for most such physical parameters there is a lack of relevant measurement techniques. For instance, a dissolution testing of a tablet may show that the active component is released too slowly. However, dissolution testing is a technique that measures indirect effects of a deviating sample batch rather than probing the physico-mechanical parameters that are the primary cause of the deviation.
The article “Analysis of gas dispersed in scattering media” from M. Sjöholm et al, Optics Letter, Vol. 26, No. 1 describes how free gas dispersed in scattering materials can be detected and characterised by use of diode laser spectroscopy. Gas detection is made possible by the contrast of the narrow absorptive feature of free-gas molecules as opposed to the small wavelength dependence of the absorption and scattering cross sections in solids and liquids. This method is, however, capable of providing information only regarding the amount of gas, i.e. free oxygen contained in the scattering medium.